July 5, 2003 Notes
Helicopter Drop Test
We finished up all of the prep work for the vehicle on
Tuesday. We welded in strapping points
to hold 600 pounds of passenger sandbags in the cabin area, and we mounted five
45 pound Olympic barbell plates on a peg at the end to simulate the weight of
the final engines, plumbing, and backup recovery system that will be on the full
size vehicle. We mounted four 2 throat
engine shells as placeholders. Total
weight is just under 2400 pounds. We
use a combination of multiple chain hoists, a palette jack, and a forklift to
move the full vehicle around and get it up on the trailer, but we did wind up
breaking one of the castor wheels that we had mounted on our tank cradle. If we wind up having to use the 1600 gallon
propellant tank (the current one is 850 gallons), we arent going to be able to
stand the vehicle up under the main girder inside our shop, which will be
On Saturday, we headed out to our test site for the drop
test. There were quite a few stares on
the road in transit
We had a few spatters of rain, and the wind occasionally
gusted to 12 knots, but we were able to perform the drop in relatively calm 6
Anna rented a big RV for the day, which was very worthwhile.
It was nice to be able to take a break
in an air-conditioned space.
5 State Helicopters arrived with a big Sikorsky for the
lifting. It was very convenient that
they were based close by, and didnt have a problem with our unusual
application (although they did have us contact the local mayor and sheriff for
explicit permission). We were very
impressed with the precision that they were able to do the lifting we were
afraid that the vehicle might get dragged or bounced on the crush cone, which
could buckle it before the test even started, but they were able to perfectly
pivot it up on the nose, and gently lift it off the ground. If we had known they were that precise, we
probably could have skipped renting the forklift truck for recovery and just
had them lower the rocket back onto the trailer after the test.
We made several 18 diameter test parachutes that were
weighted to drift at about the same rate that the full size parachute was
expected to fall. We did the test drop from
1500 AGL, under the assumption that the big vehicle would fall several hundred
feet before the main chute was fully deployed.
The landing point for the test parachute was satisfactory, so we planned
the full vehicle drop for 2000 AGL.
Neil rode in the helicopter to do the parachute releasing, and Anna hung out the side of the helicopter (with a safety strap) to get aerial footage.
We had to abort our first attempt to drop the vehicle,
because the line that we ran from the helicopter to the Sea-Catch toggle release
above the rocket had wrapped itself around the chain so many times that Neil
couldnt pull it hard enough to trigger the release. This was fixed by tying loose loops of plastic every few feet
along the chain, which kept the pull-line in place.
On the second try, the release worked perfectly. You can clearly see the naturally unstable
aerodynamics of the vehicle, as it starts to tip over almost immediately after
release. We all held our breath as it
started to fall, but the drogue immediately inflated and started pulling the
main canopy out. It was nine seconds
from release to full canopy inflation.
The opening shock was negligible, barely hitting 2Gs. For high altitude flights, we are aiming for
a 200 mph terminal velocity under the stabilizer drogue at the time of main
canopy deployment, so opening shock will be much greater then.
The wake of the main canopy is so great that the deployment drogue
just rests on the canopy during descent, without any inflation at all. The real deployment system will have a much
longer line on the drogue (because it is used for vehicle stabilization before
deploying the main), which will probably cause it to trail behind the main
chute, still inflated.
The drift was going about where we expected, but we were a
little concerned when we saw that the vehicle was oscillating +/- 13 degrees
under the canopy, which is a pretty big swing at that length. The actual landing point was unfortunately
just behind some low foliage, so we didnt get a perfect shot of it, but we did
see it hit at enough of an angle that it rolled almost back upright as it
We ran over to collapse the chute and examine the state of
the vehicle. The crush cone had buckled
right at the mounting point from the angled impact, but the vehicle looked
basically sound. None of the sandbags
in the cabin had broken open. Two of
the engine support studs were bent from when it tipped back up.
We had the helicopter pick it back up and drop it off by the
trailer, which was a lot more convenient than driving the lift truck over to
When we got it back to the shop, we pulled some things apart
to take a closer look. The bent
mounting studs unscrewed right out of their mounts, so replacing those is
trivial. We are considering adding some
more bracing below the engine plates, which would probably keep them from
bending at all. When we got the crush
cone off, we did find that the cabin had been bent right at the end of the
cone, and the buckle in the crush cone had pushed in far enough to crease the
We are probably going to continue using this cabin for the
first couple flights of the big vehicle, but start on a second-generation cabin
structure that will incorporate some improvements for off-angle landings, as
well as several other lessons we have learned in working with the current
cabin. Because we bonded a mounting
flange to the tank, we should be able to simply swap the cabin when we want to.
The accelerometer data showed 10G acceleration peaks during
the landing and bounce, which is over twice what we saw with the straight down
drop tests that collapsed perfectly.
This is still acceptable, although bouncing up and back down in the
cabin would have been a pretty harsh ride.
Making some changes to the vehicle structure will improve the behavior
of the crush cone and over tipping effects, and we are going to see if Strong
Enterprises can do anything with the canopy design to reduce the oscillations
Overall, the operation was a good success, and demonstrates
that recovering the complete vehicle after flight should work fine.
The test video is 23 megs, incorporating footage from three
cameras: one on the nose of the vehicle, one on the ground, and one in the
Images from my camera on Tuesday:
Images from Matt:
Images from Russ: